It is desirable in optical wavelength-division-multiplexing networks to have inexpensive optical modulators that have high contrast and wide optical bandwidths. One potentially suitable optical modulator is the surface-normal micromechanical optical modulator. This device has a variable air gap defined by two layers of material, one of which is vertically-movable. The movable layer is usually embodied as a membrane, the other layer is typically a substrate.
Such modulators typically have two states; an "on state" and an "off state." In the on state, a voltage is applied across the membrane and substrate generating an electrostatic force that causes the membrane to move towards the substrate. In the off state, voltage is not applied, and the membrane is quiescent. The change in membrane position relative to the substrate alters the optical properties of the device, which can be utilized to modulate an optical signal. In particular, in one of either the on state or the off state, a minimal portion of the optical energy incident upon the modulator is returned in the surface normal direction. In the other state, a significantly greater portion of the incident optical energy is so directed. Thus, the optical signal is modulated by the difference in optical energy returned in the surface normal direction in the two states.
In the absence of sufficient damping, the modulator membrane tends to ting or vibrate after moving from one state, i.e., position, to the other. Since the optical properties of the aforementioned modulators change with changes in membrane position, such ringing will affect modulator performance. In fact, the operating bit rate or frequency of such micromechanical modulators may be limited by the membrane's tendency to vibrate.
Typical prior art modulators exploit the gas, usually air, that is within the modulator cavity to provide damping. In particular, the shear flow that is generated in the air as the membrane moves dissipates the kinetic energy of the membrane, hence providing damping. At frequencies above about 1 MHz, however, this dissipation mechanism becomes ineffective because the air does not have time to flow. Rather, the air is compressed as the membrane moves downwardly, and stores energy like a spring.
As such, there is a need for a micromechanical modulator adapted to damp membrane vibrations that may occur, for example, when operating at frequencies above 1 MHz.